Well, this should be in Discussion, I think, so I will move it.
In any case, shaz is correct. I think there are two things to consider.
You don't want the trip to be boring. Just walking forever is not fun, and so it is not a bad idea to sacrifice the reality of a situation in order to make it fun. However, one way to make journeys be long and still keep the player interested are through sidequests, events, and also just put in several towns. If you've ever played Betrayal at Krondor, you will notice that while the main quests always involve a lot of walking (a big world), they make the walk more interesting by having smaller towns and small quests in between the major events of the story. Most of the towns in the game serve no major purpose in the actual story.
Also, you can use whatever size maps you want, but keep in mind that big maps with a lot of events can lag a bit. I suggest you get an anti-lag script if you choose to have one large map. The benefit of small maps is that it makes it seem less boring, like you are getting somewhere. The negative is that you leave too quickly and it can be annoying to have to go through a lot of little areas when there is nothing special to do. If you use small maps, make sure there is something that they can do there that takes at least a little time to do.
Anyway, those are just my thoughts.